Higher rates of gun ownership correlate with higher homicide rates

A new study that used state-by-state data on gun ownership and homicide rates …

A new study published in the journal Social Science and Medicine
by a team of Harvard researchers finds a strong correlation between
state-wide homicide rates and the fraction of households in a state that own firearms. While similar studies
have been carried out in the past, this one is the first to include the
entire nation and to use results available on a state-by-state level.
With two out of three homicide victims in the United States killed by
firearms, the authors of the study sought to understand what role
household firearms played in this national tragedy.

To obtain the best, most widely-spread data, the researchers used separate
data sets for gun ownership and homicides. Home firearm ownership data
was from 2001 and was collected as a part of the Behavioral Risk Factor
Surveillance System (BRFSS), a nationwide phone survey with
over 200,000 respondents that is carried out by the Centers for Disease Control
(CDC). The exact question asked of participants was:

"The next question is about firearms, including
weapons such as
pistols,
shotguns, and rifles; but not BB guns, starter pistols, or guns that
cannot fire. Are any firearms now kept in or around your home? Include
those kept in a garage, outdoor storage area, car, truck, or other
motor vehicle."

Data for homicides was compiled from mortality rates from each of the 50 states taken from the CDC's Injury Statistics Query and Reporting System Homicide data. This data was broken down into two groups: firearm related and non-firearm related; each group was further broken down into age stratifications. It was found that in less than 3 percent of the reported shootings, the intent was unknown; that data was discarded.

This problem is far more complex than simply looking at homicides and the percent of households that own firearms. In order to be thorough, the researchers controlled for a variety of factors such as the rate of aggravated assault and robbery, urbanization, unemployment, alcohol use, the percentage of the population who were between the ages of 18 and 34, percent of divorce, as well as a binary indicator of whether or not the state was in the south (as defined by the geographic region detailed in the US Census). The researchers also used a principle component analysis to generate a socioeconomic index they called the resource deprivation index (RDI)—this had previously been shown to have an invariant relationship with homicide rates across the country for multiple years. The RDI consisted of three economic and two social factors: median family income, percentage of families living below the poverty level, the Gini index of family income inequality levels, the percentage of population that is black, and the percentage of families headed by a single female.

The raw data showed that one in three U.S. households had a firearm, and unadjusted results showed a strong relation between homes with a gun and rates of firearm homicide victimization for women, but not for men. When the data was adjusted for all the control variables, an extremely strong correlation was found between states with the highest levels of homes with firearms and the number of firearm-related homicides. Indeed, states in the top 25 percent of household firearm ownership had firearm-related homicide rates that were 114 percent greater than states that had household firearm ownership in the bottom 25 percent. Overall homicide rates were a full 60 percent higher in the same states. These numbers, corrected for various factors, showed that there was no significant difference between, men, women, and people of all age stratifications. In states in the top and bottom quarters of gun ownership, the data indicates that there is no statistical difference in the rates of homicides that didn't involve guns. States with firearm prevalence more than one standard deviation above the mean include Alabama, Arkansas, West Virginia, Mississippi, South Dakota, Idaho, Alaska, Montana, and Wyoming. On the converse, states that had a firearm prevalence that was more than a standard deviation below the mean included Hawaii, Massachusetts, Rhode Island, New Jersey, Connecticut, New York, Illinois, California, and Maryland. A further analysis of the data showed that overall homicide rate and firearm-related homicide rate correlated strongest with the percent of people living in urban areas, robbery and aggravated assault rate, the resource deprivation index, and living in the south. A similar analysis applied to non-firearm homicide rates showed that the RDI, robbery and aggravated assault rate, and the divorce rate had the largest impact.

The authors are careful as to what conclusions they can draw from this work. They stress that this work does not establish a "causal relationship between guns and homicide", stating that this could be a case of "reverse causation"—that there are a higher number of households with guns because of an already high homicide rate. However, a reverse causation hypothesis would not be capable of explaining the non-correlation between household firearm prevalence and non-firearm related homicides found in the data. The authors offer the following hypothesis: "individuals who obtain firearms in an attempt to protect themselves from violence plausibly respond to non-fatal violence (which is far more common than fatal violence), the lack of association between firearm prevalence and non-lethal violent crime militates against reverse causation as an adequate explanation for our findings." Here they are saying that the idea that people obtain firearms as a means of defense in response to a violent crime that has already occurred is not a common scenario. They report that the findings are in-line with previously published results, but they also take care to point out potential shortcomings of the work. The researchers state that there are other possible factors that the study did not control for, and they state that some of their measured variables are only approximations.

These results also cannot say conclusively that firearm related homicides were carried out with guns that come from the house. Although they do suggest that their results are consistent with the idea that the ease at which a gun can be obtained is directly related to the local prevalence of household firearms, the state level resolution of data precludes a more detailed discussion of this aspect. Finally they mention that care must be taken since the home firearm prevalence data is from the year 2001, yet the homicide data was taken for the years 2001-2003. However, the researchers show that this should have little effect on the overall conclusions—that there is a higher incidence of firearm-related homicide in areas where more homes have firearms in them.

Matt Ford / Matt is a contributing writer at Ars Technica, focusing on physics, astronomy, chemistry, mathematics, and engineering. When he's not writing, he works on realtime models of large-scale engineering systems.